Particular gas, especially methane gas, is separated or recovered from gas mixture, such as natural gas, naphtha, liquefied natural gas (LNG), liquefied petroleum gas (LPG) and off gas derived from petrochemical industry as well as gas mixture (sewage gas) generated by biochemically treating sludge from sewage treatment facilities or garbage disposal facilities and is effectively reused as fuel gas and the like. Materials which selectively separate and recover methane from the above-mentioned gas mixture include gas separating membranes or absorbents composed of organic or inorganic material (see Patents Documents 1-3).
Patent Document 1 (JP 10-156157 A) discloses an aromatic polyimide gas separating membrane which is produced by heating an aromatic polyimide membrane at a temperature of 350° C. or more to crosslink, the polyimide membrane being obtained by polymerizing and imidizing an aromatic tetracarboxylic acid and aromatic diamine (see claim 1 and paragraph [0002] of Patent Document 1). The resulting membrane is used for separating and recovering specific gas from natural gas and the like. The membrane of Patent Document 1 is poor in heat resistance, durability, chemical resistance (corrosion resistance), because the membrane is composed of organic material.
Patent Document 2 (JP 2001-269570 A) discloses an absorbent for digestion gas, which is formed from an activated charcoal having a specific surface area of 800 to 2,400 m2/g, a pore volume of 0.4 to 1.5 cm3/g and a pore diameter of 7 to 20 Å (see claim 1, paragraphs [0001] to [0002] of Patent Document 2). The absorbent is used for effectively recovering/storing gas mixture (digestion gas) mainly containing methane, carbon dioxide or the like, which is generated by biochemically treating sludge from sewage treatment facilities or garbage disposal facilities. The absorbent of Patent Document 2 inevitably needs absorption and desorption steps and makes steps and facilities for separating or recovering specific gas complicated, because the material invented in Patent Document 2 is an absorbent.
Patent Document 3 (JP 2004-533927 A) discloses a gas permeation apparatus for separating purified methane from hydrocarbons higher than C1 in a feed gas mixture such as natural gas, naphtha, liquefied natural gas (LNG), liquefied petroleum gas (LPG), off gas from petrochemical industries and others, which employ a selective permeable membrane which is formed from glassy and amorphous or semi-crystalline polymer and has a glass transition temperature above an operation temperature of a gas permeation apparatus, the selective permeable member being formed from aromatic polyimide, aromatic polyether and the like (see claims 1 to 2 and paragraph [0001] of Patent Document 3). According to Patent Document 3, the selective permeable membrane should have a glass transition temperature more than the operation temperature of the gas permeation apparatus of 10 to 100° C., preferably 40 to 60° C., at which temperature water vapor and higher hydrocarbon in the membrane are not condensed, in order to ensure permeation of gas with high credibility and to effectively perform the purification (see paragraphs [0005], [0007], [0020] and the like). In addition, the selective permeable membrane of Patent Document 3 is formed from organic material, same with Patent Document 1, and is poor in heat resistance, durability, chemical resistance (corrosion resistance).
As mentioned above, organic polymer materials for selective permeable membranes are not sufficient in heat resistance, while inorganic materials are poor in flexibility and are fragile. Patent Document 4 (JP 2005-162957 A) proposes inorganic-organic hybrid materials having high proton conductivity, which is obtained by solidifying, according to a sol-gel method, a raw material mixture of a silicon alkoxide having a methacryloxyalkyl group; at least one silicon alkoxide selected from the group consisting of a trifunctional silicon alkoxide and a tetrafunctional silicon alkoxide; at least one selected from the group consisting of ethylene glycol, polyethylene glycol, polyethylene glycol monomethacrylate, polyethylene glycol dimethacrylate, polyethylene glycol monoacrylate and polyethylene glycol diacrylate; and a phosphorus compound (see claim 1 of Patent Document 4). Patent Document 4 states that the hybrid material has excellent heat resistance and chemical resistance (corrosion resistance) and is employed in an elevated temperature of 100 to 150° C., because of the matrix phase being composed of inorganic material, and that the hybrid material shows excellent flexibility because of organic phase which is integrated with the inorganic matrix phase (see paragraphs [0036] to [0037] of Patent Document 4). The invention of Patent Document 4 is related to a self-standing film having high proton conductivity in a condition of low water content, which is composed of silicon alkoxide as inorganic matrix phase forming material, ethylene glycol and the like as organic phase forming material and phosphoric acid compound, and which is used as a proton-conductive member of organic fuel cell. Accordingly, Patent Document 4 does not suggest a process for producing a methane-selective membrane which is formed on an inorganic porous support.